There are a hell of a lot of salsas in Mexican cuisine; beyond the usual green salsa verde made with miltomates (tomatillos) and fresh chiles and the usual red salsa roja made with dried chiles, jitomates (tomatoes) and garlic, there's an array of smoky sauces and dips made from all the chiles the Mesoamericans could breed, including salsas de semillas made for gloppy, soupy tacos de guisado. . . .
. . . And then there's salsa de aceite, meant for serious chileheads who want tacos made of tortillas, meat and salsa only.
It's exactly what you think it is: the Mexican equivalent of the chile oil that graces every Asian restaurant table in the county — dried chiles (de árbol, I think) fried with garlic and usually oregano in good oil, then pulsed in a blender with more oil until it looks like fiery sludge. The stuff is oddly appetizing-looking and completely addicting, especially served the way it was at Gustavo's event at the Fullerton library, with the most tender barbacoa ever.
I dubbed it “red crack” since Anita Lau (of Diary of a Mad Hungry Woman and formerly of The Orange County Register's Food Frenzy blog) calls Soho Taco's salsa verde “green crack.” I hope they keep it on the menu; if not, you know what to do: popular demand!